INTRODUCTION During early embryogenesis, a cleaving embryo produces blas- tomeres that adopt vastly different developmental fates. Single blastomeres or groups of blastomeres divide to produce clones of cells that constitute embryonic territories giving rise to specific tissues or organs in the embryo. How such territories are specified during development is a key question in embry- ology. In some species, all blastomeres are initially totipotent and their fates specified by cell-cell interactions while, in others, segregated cytoplasmic determinants restrict the devel- opmental fates of blastomeres (Davidson, 1986; Slack, 1991). Even in these ‘mosaic’ embryos, however, some cell fates are specified by inductive events mediated through cell-cell inter- actions (Kimble, 1981; Sternberg and Horvitz, 1986; Priess and Thomson, 1987; Sternberg, 1988; Nishida and Satoh, 1989; Goldstein, 1992). The highly regulative sea urchin embryo has been used for many investigations of the specification of cell fates. By the 60-cell stage the embryo is arranged into five distinct tiers along the animal vegetal axis: An1, An2, Veg1, Veg2 and the micromeres. Pluteus larvae can form after the deletion of any tier and the isolated vegetal hemispheres produce their fated structures such as gut and skeleton, and can regulate to produce structures normally derived from blastomeres of the animal half (Horstadius, 1973). In contrast, isolated animal tiers of animal halves (animal caps) produce ciliated epithelial balls described as ‘animalized’ Dauerblastulae (Horstadius, 1973) or ‘arrested in differentiation’ (Cameron and Davidson, 1991). Horstadius found that normal development can be restored by recombination of animal and vegetal fragments, and proposed that territorial specification along the animal-vegetal axis depends on the interaction of two opposing morphogenetic gradients (Horstadius, 1973). Fate mapping indicates that in S. purpuratus, founder cells for five distinctive spatial territories have been segregated by the 60-cell stage (Davidson, 1989; Cameron and Davidson, 1991). These territories are the oral and aboral ectoderm, the vegetal plate, the skeletogenic mesenchyme derived from large micromeres and the small micromeres. The aboral ectoderm 1497 Development 121, 1497-1505 (1995) Printed in Great Britain © The Company of Biologists Limited 1995 During early embryogenesis, the highly regulative sea urchin embryo relies extensively on cell-cell interactions for cellular specification. Here, the role of cellular interac- tions in the temporal and spatial expression of markers for oral and aboral ectoderm in Strongylocentrotus purpuratus and Lytechinus pictus was investigated. When pairs of mesomeres or animal caps, which are fated to give rise to ectoderm, were isolated and cultured they developed into ciliated embryoids that were morphologically polarized. In animal explants from S. purpuratus, the aboral ectoderm- specific Spec1 gene was activated at the same time as in control embryos and at relatively high levels. The Spec1 protein was restricted to the squamous epithelial cells in the embryoids suggesting that an oral-aboral axis formed and aboral ectoderm differentiation occurred correctly. However, the Ecto V protein, a marker for oral ectoderm differentiation, was detected throughout the embryoid and no stomodeum or ciliary band formed. These results indicated that animal explants from S. purpuratus were autonomous in their ability to form an oral-aboral axis and to differentiate aboral ectoderm, but other aspects of ectoderm differentiation require interaction with vegetal blastomeres. In contrast to S. purpuratus, aboral ectoderm- specific genes were not expressed in animal explants from L. pictus even though the resulting embryoids were mor- phologically very similar to those of S. purpuratus. Recom- bination of the explants with vegetal blastomeres or exposure to the vegetalizing agent LiCl restored activity of aboral ectoderm-specific genes, suggesting the requirement of a vegetal induction for differentiation of aboral ectoderm cells. These results demonstrate that differences exist in aboral ectoderm differentiation between S. purpuratus and L. pictus and suggest that the formation of a cell type may occur by alternative mechanisms in two related species. Key words: sea urchin development, Spec genes, ectoderm differentiation, animal cap SUMMARY Autonomous and non-autonomous differentiation of ectoderm in different sea urchin species Athula H. Wikramanayake 1 , Bruce P. Brandhorst 2 and William H. Klein 1, * 1 Department of Biochemistry and Molecular Biology, University of Texas, MD Anderson Cancer Center, Houston, TX 77030, USA 2 Institute of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, BC, V5A1S6, Canada *Author for correspondence